1. Field of the Invention
The present invention relates to a bundle of small diameter pitch-based carbon fibers and to a method for producing the same.
2. Description of the Related Art
In recent years, for carbon fiber having a modulus of more than 600 GPa, mainly pitch-based carbon fiber made from mesophase pitch as a raw material, with which it is easy to give a high modulus, has been produced and used. The higher the modulus of the fiber, the stiffer the fiber yarn, and therefore, the greater the occurrence of fluff or breakage of the fiber yarn at the time of handling of the fiber. Therefore, a finer diameter carbon fiber which is easier to handle has been desired.
On the other hand, when preparing composite products or their intermediates, it is desired to provide a carbon fiber with a large denier, that is, a large number of filaments, so as to reduce the number of bobbins of the carbon fiber used.
With a pitch-based carbon fiber, however, it was difficult to produce a multifilament continuous fiber composed of a fiber with an average fiber diameter of 8 .mu.m or less and with at least 1000 filaments. To produce a fine-diameter fiber by a pitch-based carbon fiber, it is necessary to produce a fine-diameter pitch fiber. Pitch fiber, however, is extremely fragile and is difficult to spin, so it was extremely difficult to spin to give 1000 or more filaments. This is considered to have been because the formation of multiple holes in the nozzle plate causes the atmospheric temperature directly under the nozzle plate to become higher at the inner circumference due to the effect of the accompanying air flow caused at the time of spinning and, further, because the speed of the accompanying air flow becomes extremely great and that flow makes it impossible for fine-diameter fiber to be stably spun. By making the number of filaments lower, it becomes somewhat easier to spin a fine-diameter pitch fiber, but the resultant pitch fiber yarn is fragile and handling in the next process was difficult.
Japanese Unexamined Patent Publication (Kokai) No. 1-229820 describes a pitch-based carbon fiber with less than 1000 filaments. In this publication, a method is disclosed in which a pitch fiber yarn with less than 1000 filaments is obtained and several of these are doubled. With less than 1000 filaments and with a fine-diameter pitch fiber for giving the carbon fiber a fiber diameter of not more than 8 .mu.m, however, the strength of the yarn is remarkably small, and therefore, it is difficult to give sufficient tension necessary for the doubling and only a carbon fiber with an insufficient fiber alignment could be obtained. Even if the greatest of care is taken and it were possible to produce a high grade carbon fiber by the doubling method, a complicated doubling process would have to be undertaken and this would make an increase in the price of the resultant carbon fiber inevitable.
To obtain the pitch fiber used as the raw material of a pitch-based carbon fiber by melt-spinning, the general practice is to provide a plurality of capillaries in the nozzle plate and extrude molten pitch from the spinning nozzle. In such an apparatus, an accompanying air flow is caused by the spun out yarn at the spinning side of the nozzle plate. Usually, the nozzle plate is disposed in an annular fashion with respect to the circular nozzle from the heat transmission surface, but in this case, the atmosphere directly beneath the nozzle plate becomes higher in temperature at the inside and lower in temperature at the outer circumference due to the effect of the accompanying air flow, which results in a large difference in the atmosphere. Therefore, it was not possible to stably spin or it was difficult to obtain a high grade fiber. Therefore, proposals have been made for blowing cooling gas at the bottom center of the spinning nozzle, attaching a mesh pipe, etc., to lower the temperature of the atmosphere at the inner circumference of the nozzle.
In particular, as a method suitable for spinning a pitch-based carbon fiber, Japanese Unexamined Patent Publication (Kokai) No. 62-231009 proposes a method of adjusting the atmospheric temperature by placing a hollow tubular body at the inside of an annular array of spinning capillaries. In a spinning nozzle having 500 capillary holes or more in a single spinning nozzle, however, it was not possible to stably spin just by adjusting the atmospheric temperature. The problem was that effect of the accompanying air flow became greater due to the formation of multiple holes and not only the atmospheric temperature directly beneath the nozzle plate, but also the speed of the air flow became extremely large. The air flow prevented stable spinning.